1. Field of the Invention
This invention relates to a phosphate glass and, specifically, to phosphate glassed coated with a film having strong resistance to water.
2. Description of the Prior Art
Heretofore, oxides have been preferentially used in contact with the glass surface in coating the glass surface. The preference for the oxide is due to its better adhesion to silicate and borate glasses than other materials. Sometimes, a fluoride has been used in contact with the surface of a silicate or borate glass, to obtain a refractive index required to prevent reflection.
Phosphate glasses are highly water soluble and, as a result, they are usually employed with a water resistant coating. When an oxide coating is used in contact with the glass surface which is the conventional practice, adhesion of the coating to the glass is weak. In particular, when the coated glass contacts water, the coating often peels from the surface, spots or stains are formed on the surface and the coating appears to be differently colored due to the light interference created by changes in the thickness of the coating. This is particularly troublesome with laser glasses where water tends to condense on the glass surface causing latent stains.